Modern electromagnetic communication and remote sensing systems are using increasingly higher frequencies. High frequencies more readily accommodate the large bandwidths required by modern high data rate communications and such sensing arrangements as chirp radar. Also, at higher frequencies the physical size of an antenna required to produce a given amount of gain is smaller than at lower frequencies. Some high frequencies are particularly advantageous or disadvantageous because of the physical transmission properties of the atmosphere at the particular frequency. For example, communications are disadvantageous at 23 GHz because of the high path attenuation due to atmospheric water vapor and at 55 GHz because of oxygen molecule absorption. On the other hand, frequencies near 40 GHz are particularly advantageous for communication and radar purposes in regions subject to smoke and dust because of the relatively low attenuation of those frequencies.
When a high gain antenna array is required, it is advantageous if each antenna element of the array has physically small dimensions in the arraying directions. For example, if it is desired to have a rectangular planar array of radiating elements for radiating in a direction normal or orthogonal to the plane of the array, it is desirable if the physical dimensions of each antenna element in the plane of the array are small so that they may be closely stacked. For those situations in which an antenna array uses a large number of radiating elements, it is also desirable that the radiating elements be substantially identical so that the radiation patterns attributable to each radiating element are identical. A prior art antenna which is useful at millimeter wave frequencies such as 40 GHz and which has relatively small dimensions in a plane normal to the direction of radiation is the finline antenna. The finline antenna consists essentially of a pair of conductive coplanar fins defining a slot therebetween. The electromagnetic energy propagates in the direction of the length of the slot, constrained in the region about the slot and between the conductive fin elements. Such a structure has characteristics of both a transmission line and of an antenna. When the width of the slot lying between the conductive fins is the same from point to point along its length, the radiation therefrom is minimal, and the transmission line properties predominate. When the width of the slot changes from point to point along its length, radiation occurs in a travelling-wave mode. Th change in width of the slot may be a linear function of distance from an origin. A special case of a finline antenna having a slot width which changes in an exponential manner is known as a Vivaldi antenna. Those skilled in the art known that the receiving and radiating properties of antennas are reciprocal, so that discussion of an antenna in terms of its transmission properties defines its receiving properties, so no explicit discussion of the receiving properties is included herein. When a large number of antenna elements are used in an antenna array, it is desirable for each of the antenna elements to have the same radiating characteristics, in order to simplify the calculation and control of beam direction. When antenna elements are intended for high frequencies such as 40 GHz, they tend to be physically small. For example, at 40 GHz, one half wavelength is 0.147 in. (3.74 mm). The small size of the antenna elements and the element-to-element repeatability required for a high gain antenna array suggests that each finline be formed by printing the conductive pattern onto the side of a dielectric substrate by the methods known for printed circuit boards or for integrated circuit substrates.
It is desirable to maximize the gain from each finline antenna element.